Control systems such as electronic engine controllers receive signals from a variety of sources for the purpose of monitoring various engine parameters. In particular, certain types of engine sensors such as certain temperature sensors and pressure sensors are operable to provide a pulse width modulated signal to the engine controller having a duty cycle which is proportional to the parameter being sensed. In such cases it is desirable to determine the duty cycle of the received pwm signal for purposes of monitoring the given parameter. In the past such pwm signals have been passed through analog filters capable of establishing a dc voltage level proportional to the duty cycle of the pwm signal. An analog to digital converter was then utilized to convert the dc voltage level to a digital value which could be read and interpreted by processing means such as a microprocessor.
One problem with such systems was the inherent error associated with analog filters, such as error associated with component variations. Further, although the engine sensors utilized are intended to output a pwm signal having a certain frequency, often times there is a certain amount of frequency drift associated with the pwm output signals of such sensors. For example, a sensor intended to output a pwm signal of 5 kHz may output a pwm signal having a frequency in the range of 3 to 12 kHz. The dc voltage level established by an analog filter could therefore vary depending upon the frequency of the pwm signal and the cutoff frequency of the analog filter. Further, the dc voltage level established could include some jitter in lower frequency pwm signals. In systems including long cables running from the sensors to the duty cycle measuring point, there can be capacitance associated with such cables which is sufficient to cause duty cycle error, and it is therefore desirable to reduce the capacitance in such systems. The capacitance can be reduced by eliminating analog filters. Still further, such prior art systems have required a significant amount of PC board space, particularly when multiple pwm signals were involved.
Accordingly, the present invention is directed to overcoming one or more of the problems as set forth above.